1. Field
The disclosure relates to a method and apparatus for switching optical filters. More specifically, the disclosure relates to a method and apparatus for providing an optical thin film system which not only prevents transmission of certain wavelengths, it also allows controlling the intensity of the transmitted wavelengths.
2. Related Art
Optical lenses and coatings which filter light have gained substantial popularity in recent years. New manufacturing processes have enabled coating multilayer optical filters on inexpensive transparent substrates and thereby flooding the market with inexpensive sunglasses capable of providing UV protection. High performance optical coatings are used in numerous commercial applications, such as camera lenses, to increase the total transmission of the camera lens. The same technological principles have been applied to window panes in order to provide optically transparent but infrared (heat) blocking windows for reducing the thermal load on buildings. The goal of most optical coatings is to provide a particular light transmission profile (transmission vs. wavelength) that yields the desired performance.
In the eye glass realm, a goal is to provide perfect optical vision while reducing the eyes' exposure to UV and other hazardous rays. In window treatments, the goal is to limit UV rays, infrared rays, and/or to limit visibility through the windows. Ultraviolet radiation falls within a range of wavelengths below visible light, generally between 100-400 nm. The ultraviolet radiation is responsible for fading and solar damage to fabrics. It is desirable to eliminate such radiation via window coatings.
Infrared radiation results in excess heating of interior spaces and is also desirable to eliminate via window coatings. Visible transmittance, on the other hand, may be desired to have different transmission levels depending on the time of day or particular application. It may be desirable for a window to have less transmission during bright daylight hours to avoid glare and eye strain while at night a window might be highly transparent to allow good outside visibility.
A particular type of optical thin film coating is the so-called rugate filter. Rugate filters are optical filters, typically applied as a coating, which act as a notch filter by eliminating a particular range of wavelengths. Rugate filters are different from discrete stacked filters in that the index of refraction of the rugate filter varies as a function of the thickness of the deposited film.
Typically, the optical thickness of the refractive index determines the reflection band position and the amplitude of the variation of the index of refraction determines the reflection bandwidth. Multiple reflection bands can be generated by depositing a series of individual layers each having an index of refraction for each reflection band. Alternatively, multiple index of refraction profiles can be superimposed and deposited in parallel. The use of superposition allows for a better film complexity without increasing the film thickness.
Conventional multilayer coatings and rugate filters are designed by applying well-developed theoretical methods to define index of refraction versus thickness profiles that result in the desired optical performance. The spectral performance of the optical coating is related to the excursion of the index of refraction and physical thickness of the coating. As such, the maximum bandwidth limited to the maximum difference in the refractive indices of the constituent materials.
Moreover, all of these optical coatings and rugate filters are not capable of adjusting for the environment's brightness or darkness. Thus, sunglasses having an optical coating are not useful at night or under variable lighting conditions.
A different type of optical filter includes electrochromic material. Electrochromic devices have been developed to change color in response to radiation or electrical stimulation. For example, electrochromic glasses change opacity through darkening upon closing a circuit which applies a charge to the electrochromic material in the glass. A typical electrochromic material uses chemical material that can reversibly change color when an electric field is applied. A common electrochromic material is aniline which reversibly changes color when a charge is applied. A common application of such electrochromic devices is in automotive rear view mirrors where they are highly reflective during daylight hours and less reflective at night to subdue reflection of headlights from following vehicles.